1. Field of the Invention
The present invention relates generally to rolls for use in a printing press. More particularly, the present invention relates to rolls and rings used in nips in a printing press.
2. State of the Art
Those skilled in the art will recognize that the press shown in FIG. 7 is an exemplar of a conventional web-fed printing press that can be configured differently in accordance with techniques and principles well known in the art. For example, the press 700 can be configured to have multiple print units 720, multiple infeed units 710, multiple folder units 750, depending on the application. In addition, the dryer unit 730 and the chill unit 740 can be omitted. Although only one dryer unit 730 and one chill unit 740 are shown, a conventional web-fed printing press can include multiple dryer units and/or multiple chill units.
In conventional web-fed printing presses, such as the press of FIG. 7, nips contact ribbons or webs of print media moving through the printing press and are often used to control and/or stabilize the webs. Nips can be used in many areas of the printing press. For example, nips can be used in the infeed unit 710, the chill unit 740, and the folder unit 750 of the printing press.
A nip is a device that catches or squeezes an object between two surfaces, points, or edges. Printing press nips typically include two cylinders positioned close together, so that the two cylinders simultaneously contact and squeeze or grip the web as it passes between them. FIG. 8 shows a side view of a nip 800 including nip cylinders 812 and 814, and a web 810 passing between the cylinders 812 and 814. If one of the nip cylinders has a diameter that varies along the rotational axis of the cylinder, then the other cylinder can have cooperating variations in diameter. Alternatively, the other cylinder can have a substantially constant diameter along its rotational axis. The web 810 need not form a straight line as it passes through the nip 800. Each of the nip cylinders 812 and 814 can be, for example, a nip roll or a nip ring. Rolls typically provide full contact along the entire width of the ribbon. Rings can also be used, and are typically movable along a shaft so they can be adjusted for different web widths, printing situations, and the like. Nip rings provide partial contact along the width of the ribbon, often near the ends of the ribbon width. Typically, at least one of the cylinders in a nip is driven. Often, one of the nip cylinders is a driven steel roll, and the other is a polymer-covered idler roll.
In particular, nip rolls and nip rings can perform multiple functions that enhance performance of the printing press. For example, they can grip the ribbons, and thus regulate both speed and tension of the ribbons as the ribbons move through the printing press. The ribbon is typically stretched during the cutting process, so that when the ribbon is cut the release of tension in the ribbon causes the cut ends to snap away from the cut point. A special kind of nip roll called an anti-snap-back (ASB) roll can be used to grip the ribbon near the cut point and thus reduce snap distances of the cut ribbon ends.
ASB rolls can also be used to corrugate a ribbon or web along the ribbon's direction of travel through the printing press, thereby providing a stiffened free end when the ribbon is cut. The increased stiffness of the cut end allows the cut end to be guided with greater precision between stages within the press. Nip rolls and/or nip rings can also be used to help set and reinforce a fold or crease in a ribbon when the ribbon passes through a nip. Typically, nip rings and ASB rolls are located within the folder unit.
Although nips that use solid steel nip rolls and/or knurled steel nip rings are typically durable and long-wearing, their performance is sensitive to the setting and adjustment of a roll-to-roll gap between the nip rolls and/or nip rings, particularly when a small number of ribbons are run through the press. Efforts have been made to redesign the roll-to-roll gap mechanism to offer better mechanical advantage and to increase the precision of the roll gap setting, but the resulting mechanisms only marginally maintain the roll-to-roll gap when the printing press is running.
Since the early 1960's, rigid nip rolls and nip rings having a thin, external layer of polymer have been incorporated into some web-fed printing presses. These rolls and rings are less sensitive to roll-to-roll gap settings than the solid steel rolls and rings, but exhibit greater wear over time. In addition, although the polymer covering is flexible, it is also incompressible. This can result in an undesirable gain in velocity of the ribbon as it moves between the rolls or through the nip, for example when the roll-to-roll gap is narrow. The gain typically increases with a magnitude of deflection or displacement of the polymer.
Other roll configurations have been disclosed, as for example in European Patent Application EP 0 743 183 A2.
It would be desirable to provide nip rolls and nip rings that are more durable than the polymer covered nip rolls and nip rings and provide constant gain independent of deflection, and that require less precise and frequent adjustment than the solid steel nip rolls and nip rings.